Nuclear magnetic resonance measuring apparatus



Sept. 12, 1961 c. w. PINKLEY NUCLEAR MAGNETIC RESONANCE MEASURINGAPPARATUS Filed Dec. 28. 1959 2 Sheets-Sheet 1 m P C FPl.| U M F a .S ER T F E A D w E P e m I. M w L C S O u m {L INVENTOR Sept. 12, 1961 c.w. PINKLEY 2,999,978

NUCLEAR MAGNETIC RESONANCE MEASURING APPARATUS Filed Dec. 28. 1959 2Sheets-Sheet 2 2 HA H8 p- LIJ con. A 0.5R con. B

DISTANCE BETWEEN COILS 7y 6 K Q IINVENTOR M 5 4 Aim United States Patent2,999,978 NUCLEAR MAGNETIC RESONANCE MEASURING APPARATUS Clyde W.Pinkley, Columbus, Ohio, assignor to Industrial Nucleonics Corporation,a corporation of Ohio Filed Dec. 28, 1959, Ser. No. 862,361 '10 Claims.(Cl. 324-.5)

This invention relates to measuring apparatus, and in particular to animproved magnet and coil assembly for conditioning flat sheet materialto be analyzed by measurement of various phenomena occurring in responseto nuclear magnetic resonance.

It is well known in the prior art relating to nuclear physics that manyatomic nuclei possess magnetic moment and nuclear momentum or spin. Anucleus having these characteristics displays gyroscopic eifects and istherefore often considered analogous to a spinning gyroscope having amagnet positioned along its axis.

When such nuclei are subjected to a constant uniform external magneticfield, the spinning nuclei tend to precess around an axis parallel tothe magnetic field with a characteristic frequency and with randomphase. Initially the resulting polarization is zero, but, after acharacteristic time, damping forces cause an excess of nuclei to exhibita magnetic component in the direction of the external field. In theevent the polarized nuclei are subjected to a radio-frequency magneticfield at right angles to the external field and at the frequency ofnuclear precession, phase coherence is introduced among the nuclei withthe result that the induced polarization precesses about the externalfield direction with a corresponding loss of energy in theradio-frequency field.

Prior investigators have studied the gyroscopic properties of nuclei bysubjecting an element to a magnetic field produced by a permanent magnetor electromagnet and simultaneously irradiating the element withradio-frequency electromagnetic energy emanating from a tank coil.

When the frequency of the radio-frequency source resonates with thefrequency of nuclear precession, the spinning nuclei absorb a maximumamount of energy from the radio-frequency field thereby loading the tankcircuit. It has been determined that the resonant frequency of nuclearprecession varies for different elements and for different values of thepolarizing magnetic field.

Within recent years, measuring devices have been proposed operative inresponse to the energy absorption occurring at the nuclear magneticresonance frequency. From this absorption measurement, the relativeproportion of an element in question can be determined because the totalenergy absorbed is a function of the number of nuclei present, otherthings being equal. Apparatus of this type can be used for thequantitative determination of any element the nucleus of which possessesangular momentum and magnetic moment, such as for example, H H He Li LiBe B B N etc. Additionally, quantitative determination of numerousisotopes of elements can also be made, because in all cases thediiferent isotopes possessing non-zero magnetic moments have dilferentresonant frequencies in the same external field.

The absorption phenomenon of nuclear magnetic resonance is also used tomeasure constituent proportions in various compounds. For example,moisture content measurements can be made in materials, such as tobaccoor paper. In such a determination the water content is not measureddirectly but, rather, the hydrogen in the material is distinguished fromthe hydrogen in water on the basis of widely difiering absorptionpatterns. By applying the same principles it is possible to measure thepresence of any compound which contains at least one ele- Patented Sept.12, l96l ment the nucleus of which possesses angular momentum andmagnetic moment.

In one type of conventional nuclear magnetic resonance apparatus,radio-frequency current from a constant-current source is supplied to aparallel tuned circuit consisting of a coil and capacitor. The tankcoil, ordinarily a solenoid is placed within the uniform external fieldof a permanent magnet so that the radio-frequency field is perpendicularto the external field, and the material to be measured is placed withinthe coil.

The frequency of the radio-frequency field, or the magnitude of theexternal field, is modulated at a slow audio rate. When theradio-frequency and the magnetic fields satisfy the relation W ='yHwhereW is the angular velocity of the radio-frequency field H H is thepermanent magnetic field strength in gauss, and y is a constantdependent on the type of nucleus subjected to resonance, nuclearmagnetic resonance occurs.

The resulting nuclear resonance causes a decrease in the impedance ofthe tank circuit, and therefore a decrease in the voltage appearingacross the tank circuit. For a given set of conditions the magnitude ofthis change in voltage is proportional to the amount of absorbingsubstance present so that a quantitative measurement can be made.

With a given amount of absorbing substance, the magnitude of voltagechange is proportional to the radiofrequency field strength providedthat saturation does not occur. It is therefore desirable to maintainthe field strength to as high a value as possible without attainingsaturation.

The use of nuclear magnetic resonance for the measurement of moisture orother characteristics of sheet material presents the problem ofsubjecting a cross section of the sheet to the mutually perpendicularradio-frequency and external fields which satisfy the requirements fornuclear resonance. Since it is physically impossible to place a largesheet of material within a radio-frequency coil of the solenoidal typewhere the field has its highest, most uniform concentration, the sheetin many prior art arrangements is subjected to the field extending fromthe end of the coil (where the field is dispersed) thus greatlydecreasing the sensitivity of the measuring device.

The disposition of the pole pieces required to generate a magnetic fieldH perpendicular to the radio-frequency field H has also dictated severalcompromise magnet and coil assemblies Which have resulted in relativelypoor signal-to-noise ratios in the measurement of sheet material.

Additionally, optimum signal-to-noise ratios and accurate measurementsare obtained when the largest possible portion of the totalradio-frequency field H is placed in the sheet portion under test, or inother words a high filling factor is desired. Another importantrequirement is to obtain a radio-frequency field H that is reasonablyuniform with respect to possible changes in the relative position of thetest sample. In the case of a continuous flat sheet, this undesirablechange in relative position is,

attained by movement in a direction perpendicular to the sheet. Magnetand coil assemblies heretofore employed have not provided fullysatisfactory measurements in sheet material because of the inability toadequately meet these requirements.

Accordingly, a principal object of this invention is to:

improve the signal-to-noise ratio and the sensitivity of nuclearmagnetic resonance sheet measuring apparatus.

Another object is to provide an improved magnet and coil assembly fornuclear magnetic resonance measuring apparatus that is advantageouslyadapted for the testing of sheet material.

Another object is to provide a magnet and coil assembly of substantiallyimproved efiiciency for nuclear magnetic resonance measuring apparatusthatsubjects the material under test to higher external fieldintensities of better homogeneity.

A preferred embodiment of the magnet and coil assembly of this inventioncomprises a modified Helmholtz pair of coils disposed between a pair ofspaced magnet pole pieces. The coils are axially aligned and closelyspaced in a parallel relationship with respect to one another. Bothcoils are included in a resonant tank circuit that develops an outputsignal in response to a condition of nuclear magnetic resonance in sheetmaterial positioned between the coils.

The coils are connected in -a reverse sense so that their respectiveradio-frequency magnetic fields are in opposition or bucking. Thisconnection provides radio-frequency magnetic flux between the two coilswhich is essentially radial to the common axis of the two coils exceptat points near the axis and beyond the outer perimeter of the coils. Thesample sheet under test is preferably disposed in the median planebetween the coils that is perpendicular to the common axis of the coils.The resulting lines of flux emanating from the two coils thus lie in theplane of the material under test.

Due to the increased flux density between the coils a satisfactoryfilling factor is obtained with coil spacings of a few samplethicknesses.

A principal novel feature of this invention is directed to theconstruction details for both coil windings. The distance betweencorresponding turns on the two coils is equal to the radius of theturns. With this geometry, the rate of change of magnetic field withdistance from the plane of a given coil is a constant at a point equalto one-half the radius of the coil. .Inasmuch as two coils are used, oneon each side of this point, the de crease in field of one coil isbalanced by the increase in field of the other so that the combinedfield will be very nearly constant. The primary improvement overprevious fiat coil designs is the improvement in radiofrequency fielduniformity between the coils and the increase in size of the uniformregion for a given coil size.

In order that all of the features for attaining the objects of thisinvention may be readily understood, reference is herein made to thedrawings wherein:

FIG. 1 is a simplified circuit diagram of nuclear magnetic resonancemeasuring apparatus employing the improved magnet and radio-frequencycoil assembly of this invention;

'FIG. 2 is a perspective view showing the improved arrangement herein ofthe magnet pole pieces and the radio-frequency coils relative a sheet ofmaterial under test;

FIG. 3 is a cross-sectional view of the magnetic and radio-frequencyfields generated by the radio-frequency coils of FIG. 2;

FIG. 4 is a diagram of the radial components of the radio-frequencyfield generated in the view of FIG. 3;

FIG. 5 is a diagram of assistance in showing the relationship betweenthe radius of the coil turns and the distance separating correspondingcoil turns; and

FIG. 6 is a set of curves showing various field strength curves for apair of single turn coils designed in accordance with the requirementsof FIG. 5.

Referring now to the simplified showing of the measuring apparatus ofFIG. 1 incorporating the magnet and coil assembly of this invention,sheet material 10 under test is positioned midway betweenradio-frequency sampling coils 11 and 12. The coils 11 and 12 areaxially aligned and disposed in a parallel relationship with respect toone another. For reasons hereinafter outlined in detail, material 10 issubjected to a radio-frequency field H radial to the axis of the coilsand within the plane containing the sheet. Material 10 is also subjectedto a magnetic field H developed in the gap between external magnets 13and 14. Field H is perpendicular to the radial components of theradio-frequency field H Modulation coils 15 and 16 envelope the poleends of magnets .13 and 14, respectively, so that the otherwise steadymagnetic field is amplitude modulated by the audio-frequency energysupplied from modulation source 17.

Capacitor 18 shunts coils 11 and 12 so that the combination 11, 12, 18forms a parallel-resonant tank circuit connected to the output ofconstant-current radiofrequency oscillator 19. Other circuitarrangements may be apparent such as connecting coils 11 and 12 inparallel with the connection to the condenser 18 either in series or inparallel. Alternately the coils 11 and 12 may @be independentlyresonated with two separate condensers and coupled together in anopposing sense by means of identical transmission lines from a commonradio-frequency source properly matched in impedance to the source andto the resonant circuits.

The tank circuit is tuned to the oscillator frequency and therefore asubstantial radio-frequency voltage appears across the tank circuit.This voltage has a constant amplitude except during those periodicinstances at which the output frequency of oscillator 19 and themodulated magnetic field generated by the magnets 13 and14 andmodulation coils 15 and 16 satisfy the requirements for nuclearresonance.

During resonance, material 10 absorbs energy from the radio-frequencyfield so 'as to periodically load coils 11 and 12. As is well known, theloading of a parallel tank circuit lowers the Q of the tank, therebyreducing the parallel impedance appearing across the tank circuit. Theperiodic absorption of energy by material It thus amplitude modulatesthe radio-frequency voltage appearing across tank circuit 11, 12, 18.The am plitude of this modulation component varies in accordance with.the number of nuclei present in the material 10 to absorb energy fromtank coils 11 and 12.

The voltage appearing across tank circuit 11, 12, 18 is applied to theinput of radio-frequency amplifier 20. The signal output ofradio-frequency amplifier 20 is in turn applied to the input of detectorand audio-frequency amplifier 21. This latter unit develops anaudio-frequency signal corresponding to the modulation componentintroduced'by absorption variations in material 10. This signal isapplied to the vertical amplifier of oscilloscope 22.

The horizontal sweep of oscilloscope 22 is synchronized to the periodicvertical voltage pulses by applying a voltage from modulation source 17to appropriate horizontal sweep terminals of the oscilloscope.Accordingly, a stationary pulse appears on the screen of theoscilloscope which has an amplitude responsive to variations in theabsorption characteristics of thematerial 10 under test.

An enlarged perspective view of the magnet and coil assembly of thisinvention is shown in FIG. 2. Sheet 10 is disposed midway between thefaces of the magnet pole pieces 13 and 14 and also midway between theadjacent end surfaces of radio-frequency coils 11 and 12. The sheetmoves in a plane parallel to the adjacent faces of the magnet polepieces and also the adjacent surfaces of radio-frequency 'coils 11 and'12. Conventional means may be employed for moving sheet 10 throughoutthe required plane, the essential requirement being that the sheetpreferably be maintained in the relative position set forth.

Radio-frequency coils 1 1 and 1.2 are spiral-wound as hereinafteroutlined in detail so that end openings 25 and 26 are formed therein,respectively. The two coils 11 and 12 may both be real physical coils asshown or one of the coils and its associated radio-frequency field-maybe an electricalimage of the other real physical coil, the electricalimage being formed by the well known method of placing a metallicconducting sheet in the position of the median plane. In this case thesample sheet must be slightly displaced from the median plane in thedirectionof the physical coil.

It should .be noted that coils 11 and Hate reversely connected inasmuchas conductor 27 connects the upper center output lead of coil 12 withthe corresponding lead of coil 11. Output connections to both reverselyconnected coils 11 and 12 are made to the outer conductors 28 and 29.Capacitor 18 is connected across conductors 28 and 29 so that a parallelresonant tank combination 11, 12, "18 is formed.

The steady magnetic field H generated by magnet pole pieces 13 and 14 isparallel to the common longitudinal axis 35 of coils 11 and 12 andperpendicular to the surfaces of sheet 10. Modulation coils 15 and 16are wound around the terminal ends of the pole pieces 13 and 14,respectively, so that the otherwise steady magnetic field H may beamplitude modulated.

FIG. 3 shows a diagram of the fields H and H generated by the magnetpole pieces 113 and 14 and radiofrequency coils 11 and 12. The X-markedconductors '30 of coil 11 and the X-marked conductors 31 of coil 12 arethose conductors in which current flows into the plane of the figure,and the plain conductors 32 of coil 11 and the plain conductors 33 ofcoil 12 are those conductors in which current flows out of the plane ofthe figure. The conductor group 30, therefore, generates clockwise fluxloops and conductor group 33 generates counterclockwise flux loops.Accordingly, the lowermost flux lines generated by conductor group 30and the uppermost flux lines generated by conductor group 33 combine todevelop reinforcing resulting flux lines which are parallel to the planeof sheet 10 and directed to the left of the figure.

Conductor group 32 generates counter-clockwise flux loops and conductorgroup 31 generates clockwise fiux loops. Accordingly, the lowermost fiuxlines generated by group 32 and the uppermost flux lines generated bygroup 31 combine to form reinforcing resulting lines which are in theplane of sheet 10. These resulting lines are directed to the right ofthe figure.

The foregoing magnet and coil assembly thus generates radio-frequencyflux between the two coils 11 and 12 which is essentially radial to thecommon axis 35 of the two coils as is shown in FIG. 4, except at pointsnear the axis 35 of the two coils and beyond the outer perimeter.

As is best shown in FIG. 5, coils 11 and 12 are identical and are sowound that the distance between corresponding turns on the two coilsequals the radius of the turns. For example, the individual turns oncoils 11 and 12 having a radius of M1 are separated from one another bya like distance M1. Likewise, the individual turns of coils 11 and 12that have a radius M2 are separated from one another by the distance M2.

For a single turn coil, the rate of change of magnetic field strengthwith distance from the plane of the coil is a constant at a point equalto one-half the radius of the coil. In the event two coils are used, oneon each side of the point, the decrease in field of one coil will bebalanced by the increase in field of the other so that the combinedfield will be very nearly constant.

The above relationship is shown in the graph of FIG. 6 wherein therelative field strengths for two identical single turn coils A and B areshown plotted with respect to the distance between the coils infractions of one radius. The coils are located in parallel planes spacedat a distance of one radius. Curve HA shows that the magnetic fieldstrength is a maximum in the plane of coil A, and that this fieldstrength varies at a constant, decreasing value at a distance ofapproximately 0.5 radius from coil A. Likewise, curve HB shows that themagnetic field strength of coil B is at a maximum in the plane of coilB, and this field strength decreases at the same rate that the fieldstrength of coil A increases when measured at a distance of 0.5 radiusfrom coil B.

Accordingly, it may be readily seen that the decrease in field strengthof one coil is balanced by the increase in the field strength of theother coil. The combined field strength for both coils A and B is shownin curve HA+HB. In view of the fact that this composite curve 6 isrelatively fiat in the region, from say, 0.3 to 0.7 radius, variationsin the position of a sheet under test between magnet pole pieces withinthis radius range will not be accompanied by variations inradio-frequency field strength.

Accordingly, the particular design of coils 11 and 12 shown herein givesvery good radio-frequency field uniformity throughout the region betweenthe coils and allows more movement of the sheet in this region withoutaffecting the sensitivity. In addition to the improvement inradio-frequency field uniformity between coils 11 and 12, there is alsoa corresponding increase in the size of the uniform region for a givencoil size.

It should be understood that the above described arrangements are merelyillustrative of the features of this invention and that numerous otherarrangements may be devised without departing from this scope of theinvention.

What is claimed is:

1. In nuclear magnetic resonance measuring apparatus for subjecting flatsheet material to be analyzed to mutually perpendicular magnetic andradio-frequency fields including a resonant tank circuit developing anoutput signal responsive to a condition of nuclear resonance between thefields and for the material under measurement, the improvementcomprising a pair of identical coils axially aligned and spaced withrespect to one another so that the distance between corresponding turnson the two coils equals the radius of the turns, a radiofrequency sourceenergizing said coils to produce opposing radio-frequency magneticfields in the common space therebetween and as measured along the axis,means adapted to support the fiat sheet material under measurement inthe space between the coils, and a pair of spaced magnet polesdeveloping a second magnetic field passing through said coils andparallel to the axis thereof.

2. In nuclear magnetic resonance measuring apparatus for subjecting flatsheet material to be analyzed to mutually perpendicular magnetic andradio-frequency fields including a resonance tank circuit developing anoutput signal responsive to a condition to nuclear resonance between thefields and for the material under measurement, the improvementcomprising a pair of substantially identical coils axially aligned andspaced with respect to one another so that each coil is an image of oneanother in a median plane therebetween, both of said coils beingincluded in said resonant tank circuit and wound so that correspondingturns on both coils have an increasing radius as the distance thereoffrom the median plane is increased, a radio-frequency source energizingsaid coils to produce opposing radio-frequency magnetic fields in thecommon space therebetween and as measured along the axis, means adaptedto support the fiat sheet material under measurement midway between thecoils and perpendicular to the axis thereof, and a pair of spaced magnetpoles developing a second magnetic field passing through said coils andparallel to the axis thereof.

3. In a measuring system substantially as set forth in claim 2 whereinone of said coils is a real physical coil and the other is meansproviding an electrical image of the physical coil.

4. In a nuclear magnetic resonance measuring apparatus for subjecting amaterial to be analyzed to mutually perpendicular magnetic andradio-frequency fields including a resonant tank circuit developing anoutput signal responsive to a condition of nuclear resonance between thefields and for the material under measurement, the improvementcomprising a pair of substantially identical coils disposed on a commonaxis and connected relative one another to develop opposing magneticfields in the space between the coils and generating reinforcingresulting lines of magnetic flux radially emanating from said axis, eachof said coils being wound so that the distance between correspondingturns on the two coils equals the radius of the turns, -aradio-frequency Source energizing both of said coils, means adapted -.tosupport the material under measurement in the spacetbetween the coils,and a pair of spaced magnet poles developing a second magnetic fieldpassing through said coils and parallel to the common axis thereof.

5. In a measuring system substantially as set forth in claim 4 whereinone of said coils is a real physical .coil and the other is meansproviding an electrical image of the physical coil.

6. In nuclear magnetic resonance measuring apparatus for subjecting amaterial to be analyzed to mutually perpendicular magneticandradio-frequency fields including a resonant tank circuit developingan output voltage responsive to a condition of nuclear resonance betweenthe fields and for the material under measurement, the improvementcomprising a pair of identical coils each formed with a center openingand included in said resonant tank circuit, said coils being in a spacedrelationship with respect to one another and symmetrically disposed on acommon axis passing through the center of said coils, each of saidcoils'being wound so'that the distance between corresponding turns onthe two coils equals the radius of the turns, a radio-frequency sourceenergizing said coils to produce opposing radio-frequency magneticfields in the common space therehetween having reinforcing resultingcomponents radially emanating from said axis, means adapted to supportthe material under measurement in the space between the coils, and apair of spaced magnet poles developing a second magnetic field passingthrough said coils and parallel to the axis thereof.

'7. In nuclear magnetic resonance measuring apparatus for subjectingfiat sheet material to' be analyzed to.mutually perpendicular magneticand radio-frequency fields including a resonant tank circuit developingan output voltage responsive to a condition of nuclear resonance betweenthe fields and for the material under measurement, the improvementcomprising a pair of identical coils each formed with a center openingand including in said resonant tank circuit, said coils being in .aspaced relationship with respect to one another and symmetricallydisposed on a common axis passing through the center of said coils, sothat each coil is an image of one another in a median planetherebetween, each of .said coils being wound so that correspondingturns on both coils have an increasing radius as .the distance thereoffrom the median plane is increased, a radio-frequency source energizingsaid coils to produce opposing radiofrequency magnetic fields in thecommon space there between having reinforcing resulting componentsradially emanating from said axis, means adapted to support the sheetmaterial under measurement midway between the coils and perpendicular toaxis, and a pair of spaced magnet poles developing a second magneticfield passing through said coils and parallel to the axis thereof.

8. In nuclear magnetic resonance measuring apparatus for subjecting amaterial to be analyzed to mutually perpendicular magnetic andradio-frequency fields, including a resonant tank circuit developing anoutput signal responsive to a condition of nuclear resonance between thefields and for the material under measurement, and having a pair ofidentical coilsaxially aligned and spaced 8 r r so that each coil is animage of one another, a radiofrequency source energizing said coils toproduce opposing radio-frequency magnetic fields in the commonspacetherebetween and as measured along the axis, means adapted to supportthe material under measurement in the space between the coils, and apair of spaced magnet poles developing a second magnetic field passingthrough said coils and parallel to the axis thereof, the improvement.comprising each of said coils being wound so that corresponding turnson both coils have an increasing radius as the distance thereof from themedian plane is increased.

9. In nuclear magnetic resonance measuring apparatus for subjecting amaterial to be analyzed to mutually perpendicular magnetic andradio-frequency fields, including a resonant tank circuit developing anoutput signal responsive to a condition of nuclear resonance between thefields and for the material under measurement, a pair of coils disposedon a common axis and connected relative one another to develop opposingmagnetic fields in the space between the coils and generatingreinforcing resulting lines of magnetic flux radially emanating fromsaid axis, a radio frequency source energizing both of said coils, meansadapted to support the coils, and a pair of spaced magnet polesdeveloping a second magnetic field passing through said coils andparallel to the common axis thereof, the improvement comprising each ofsaid coils being wound so that corresponding turns on both coils have anincreasing radius as the distance thereof from the median planetherebetween .is increased.

10. In nuclear magnetic resonance measuring apparatus for subjecting amaterial to be analyzed to mutually perpendicular magnetic andradio-frequency fields, including a resonant tank circuit developing anoutput signal responsive to a condition of nuclear resonance between thefields and for the material undermeasurement, a pair of coils disposedon a common axis and connected relative one another to develop opposingmagnetic fields in the space between the coils and generatingreinforcing resulting .lines of magnetic flux radially emanating fromsaid axis, a radio frequency source energizing both of said coils, meansadapted to support thematerial under measurement in the space betweenthe coils, and a pair of spaced magnetpoles developing a second magneticfield passing through said coils and parallel to the common axisthereof, the improvement comprising each of said coils being wound sothat the distance between corresponding turns on the two coils equalsthe radius of the turns.

References Cited in the file of this patent FOREIGN PATENTS GreatBritain Nov. 24, 1932 France Mar. 18, 1957 OTHER REFERENCES

